To induce antigen-specific T cell activation and clonal expansion, two signals provided by antigen-presenting cells (APCs) must be delivered to the surface of resting T lymphocytes (Jenkins, M. and Schwartz, R. (1987) J. Exp. Med. 165, 302-319; Mueller, D. L., et al. (1990) J. Immunol. 144, 3701-3709; Williams, I. R. and Unanue, E. R. (1990) J. Immunol. 145, 85-93). The first signal, which confers specificity to the immune response, is mediated via the T cell receptor (TCR) following recognition of foreign antigenic peptide presented in the context of the major histocompatibility complex (MHC). The second signal, termed costimulation, induces T cells to proliferate and become functional (Schwartz, R. H. (1990) Science 248, 1349-1356). Costimulation is neither antigen-specific, nor MHC restricted and is thought to be provided by one or more distinct cell surface molecules expressed by APCs (Jenkins, M. K., et al. (1988) J. Immunol. 140, 3324-3330; Linsley, P. S., et al. (1991) J. Exp. Med. 173, 721-730; Gimmi, C. D., et al., (1991) Proc. Natl. Acad. Sci. USA. 88, 6575-6579; Young, J. W., et al. (1992) J. Clin. Invest. 90, 229-237; Koulova, L., et al. (1991) J. Exp. Med. 173, 759-762; Reiser, H., et al. (1992) Proc. Natl. Acad Sci. USA. 89, 271-275; van-Seventer, G. A., et al. (1990) J. Immunol. 144, 4579-4586; LaSalle, J. M., et al., (1991) J. Immunol. 147, 774-80; Dustin, M. I., et al., (1989) J. Exp. Med. 169, 503; Armitage, R. J., et al. (1992) Nature 357, 80-82; Liu, Y., et al. (1992) J. Exp. Med. 175, 437-445). One costimulatory pathway involved in T cell activation involves the molecule CD28 on the surface of T cells. This molecule can receive a costimulatory signal delivered by a ligand on B cells or other APCs. Ligands for CD28 include members of the B7 family of B lymphocyte activation antigens, such as B7-1 and/or B7-2 (Freedman, A. S. et al. (1987) J. Immunol. 137, 3260-3267; Freeman, G. J. et al. (1989) J. Immunol. 143, 2714-2722; Freeman, G. J. et al. (1991) J. Exp. Med 174, 625-631; Freeman, G. J. et al. (1993) Science 262, 909-911; Azuma, M. et al. (1993) Nature 366, 76-79; Freeman, G. J. et al. (1993) J. Exp. Med. 178, 2185-2192). B7-1 and B7-2 are also ligands for another molecule, CTLA4, present on the surface of activated T cells, although the role of CTLA4 in costimulation is unclear.
Delivery of an antigen-specific signal with a costimulatory signal to a T cell leads to T cell activation, which can include both T cell proliferation and cytokine secretion. In contrast, delivery of an antigen-specific signal to a T cell in the absence of a costimulatory signal is thought to induce a state of unresponsiveness or anergy in the T cell, thereby inducing antigen-specific tolerance in the T cell.
Interactions between T cells and B cells play a central role in immune responses. Induction of humoral immunity to thymus-dependent antigens requires "help" provided by T helper (hereafter Th) cells. While some help provided to B lymphocytes is mediated by soluble molecules released by Th cells (for instance lymphokines such as IL-4 and IL-5), activation of B cells also requires a contact-dependent interaction between B cells and Th cells. Hirohata et al., J. Immunol., 140:3736-3744 (1988); Bartlett et al., J. Immunol., 143:1745-1754 (1989). This indicates that B cell activation involves an obligatory interaction between cell surface molecules on B cells and Th cells. The molecule(s) on the T cell therefore mediates contact-dependent helper effector functions of T cells. A contact-dependent interaction between molecules on B cells and T cells is further supported by the observation that isolated plasma membranes of activated T cells can provide helper functions necessary for B cell activation. Brian, Proc. Natl. Acad. Sci. USA, 85:564-568 (1988); Hodgkin et al., J. Immunol., 145:2025-2034 (1990); Noelle et al., J. Immunol., 146:1118-1124 (1991).
A molecule, CD40, has been identified on the surface of immature and mature B lymphocytes which, when crosslinked by antibodies, induces B cell proliferation. Valle et al., Eur. J. Immunol., 19:1463-1467 (1989); Gordon et al., J. Immunol., 140:1425-1430 (1988); Gruber et al., J. Immunol, 142: 4144-4152 (1989). CD40 has been molecularly cloned and characterized. Stamenkovic et al., EMBO J., 8:1403-1410 (1989). A ligand for CD40, gp39 (also called CD40 ligand or CD40L) has also been molecularly cloned and characterized. Armitage et al., Nature, 357:80-82 (1992); Lederman et al., J. Exp. Med., 175:1091-1101 (1992); Hollenbaugh et al., EMBO J., 11:4313-4319 (1992). The gp39 protein is expressed on activated, but not resting, CD4.sup.+ Th cells. Spriggs et al., J. Exp. Med., 176:1543-1550 (1992); Lane et al., Eur. J Immunol., 22:2573-2578 (1992); Roy et al., J. Immunol., 151:1-14 (1993). Cells transfected with the gp39 gene and expressing the gp39 protein on their surface can trigger B cell proliferation and, together with other stimulatory signals, can induce antibody production. Armitage et al., Nature, 357:80-82 (1992); Hollenbaugh et al., EMBO J., 11:4313-4319 (1992).